This invention relates to a method and apparatus for dispensing slurries of material having substantial suspended solids. More particularly it relates to a method and apparatus for dispensing lime slurries for the treatment of water.
Lime has been used for many years to treat both drinking water and waste water, primarily for the purpose of increasing the alkalinity of the water, i.e. neutralizing the acidity in the water to reduce damage to water handling equipment associated with low pH's. As the acidity in rainwater has increased throughout the industrialized world the importance of the use of lime for neutralizing the acid has increased.
Lime which is utilized in water treatment comes in two forms, namely quick lime, which is normally calcium oxide, and hydrated lime, which is normally calcium hydroxide. Calcium oxide is a white coarse solid which has a cubic crystalline structure and is made by heating high quality calcium carbonate. Calcium hydroxide is prepared by heating calcium oxide and by adding a small amount of water thereto, which forms a more stable hexagonal crystalline structure.
Calcium hydroxide or hydrated lime, which is the preferred form used in connection with systems for raising the pH of water, is slightly soluble in water but makes a suspension easily due to the small particle sizes. The resulting suspension and solution is strongly alkaline. Freshly slaked lime, that is, dry lime to which has been added a small amount of water consists of small particles which are about 10% more soluble in water than coarse particles or aged slaked lime. Commercial lime mixtures in water vary widely in regard to the settling rate of suspended particles. In general, one half of the particles will settle out in 90 minutes under laboratory conditions while the mixture is unagitated. As lime is slurried in a water mixture, the higher the amount of suspended solids the more difficult it is to achieve an even suspension. The accepted maximum lime slurry is 45% suspended solids which exhibits a thick, muddy texture. Normally a 30-35% suspended solid slurry has reasonable fluidity to allow transport and agitated storage.
Several factors affect the solubility of time in water. These factors include the amount of dissolved CO.sub.2 in the water, the temperature of the water, and particle size of the lime. Since dry lime will react with moisture in the air, a slow reaction will occur between the lime and CO.sub.2 in the air, forming water insoluble carbonates. Lime suspended in water will react with both bicarbonates and dissolved CO.sub.2 to form more carbonates. Because of the ready formation of these carbonates, lime in a slurry form is very difficult to handle in a metering or dispensing system because of the scaling which occurs inside of the tanks and feed pipes due to the formation of calcium carbonate.
Piping lime slurry presents substantial challenge to the user. To avoid settling of the lime in pipes, the lime must travel from 2.5 to 8 feet per second depending on particle size. Because of lime's inherent pH of above 12, the water that carries the lime undergoes a softening action which precipitates fresh calcium carbonate as a dense hard scale. When this formation of calcium carbonate occurs in an undisturbed mass of settled lime, crystallization among the closely packed particles binds them together in a rigid mass that clings tenaciously to solid material with which it was in contact during the reaction, such as, for example, the walls of pipes. If unattended or poorly engineered, the scale will ecrete to the point that the lime will close the pipe. Thus blockage of openings and passageways occurs if the slurry is not kept in a constant motion.
The types of piping for feeding lime slurry is quite restrictive. Since lime will react with water and CO.sub.2 forming calcium carbonate scale, feed lines must be designed to avoid scale buildup. The poorest feed lines are those of metal because interior rough surface areas found in metal pipes provide numerous sites for ecretions and thus scaling to begin. Preferred pipes are flexible plastic lines, such as PVC, with few or no elbows. These types of lines are smoother than metal and avoid caustic reactions which occur with certain metals. Also flexible plastic lines alloy for scaling to be externally removed by squeezing the pipe is a buildup begins. Elbows should be held at a minimum because of reduced flow rate as lime solids turn the corner of the elbow. The diameter of the piping is also important since smaller diameter pipes tend to clog more readily. In fact, it is commonly believed that small diameter pipes, i.e. in the 3/4" range, cannot be readily used to meter or dispense lime slurry effectively without undue clogging.